arcpy - How do you use folders as input/output parameters

What I want to do is to go into a folder, find a layer file, pull it into an MXD, then export a jpeg in an output folder. Everything is great except I don't know how to set the input/output folders.

state =  arcpy.GetParameterAsText(0)
In_Folder = arcpy.GetParameterAsText(1)
Out_Folder = arcpy.GetParameterAsText(2)


lyrfile_o = arcpy.mapping.Layer(In_Folder + str(state) + "_O.lyr")

lyrfile_d = arcpy.mapping.Layer(In_Folder + str(state) + "_D.lyr")
lyrfile_lane = arcpy.mapping.Layer(In_Folder + str(state) + "_Lanes_.lyr")
lyrfile_poly = arcpy.mapping.Layer(In_Folder + str(state) + "_Lanes_Polys1.lyr")

and I get an error like:

: Object: CreateObject Layer invalid data source>

It's written in a text editor then brought into a toolbox and parameters set, both types as folders, the first one (state) is a string.

Any ideas?